Where are the Dark Menhirs during the Grandmaster Battle?

Dark Menhirs never 1HK. During the JQ, they always bring your HP down to 100 HP. If your HP is lower than 100 when you get hit, you'll die. You can have 105 HP, get hit by one, and it'll bring your HP down to 100.
I'm ranged, so I don't know if the Dark Menhirs she summons do the same damage, but I know they don't 1HK.
You're thinking about the regular Menhirs in the Valley of Heroes.

This one she summons is actually a monster, those in the JQ are just map objects. big difference despite how they may look.